1. Field of the Invention
The present invention relates generally to the generation of focused electron beams and more particularly, to a radio frequency (RF) modulated electron gun.
2. Description of the Related Art
Electron guns have been used for decades to provide a source of electrons for devices such as linear accelerators. For example, electron guns can be used as a source of charged particles for linear accelerators used in medical radiation treatment, radiation processing of materials and other applications such as basic or applied research. For many applications, the electron gun is used to generate charged particles for input to a RF accelerator waveguide. The accelerator waveguide receives the input charged particles and accelerates them to produce an accelerated output beam of a desired frequency for use in a particular application.
Frequently, the beams generated by the electron gun are passed through one or more devices referred to as xe2x80x9cpre-bunchersxe2x80x9d which may be formed as separate chambers positioned between the electron gun and an accelerator waveguide, or may be formed as a separate input cavity of the waveguide. These devices are used to group charged particles from the electron gun into xe2x80x9cbunchesxe2x80x9d or xe2x80x9cmacropulsesxe2x80x9d of charged particles. Typically, each macropulse has a pulse duration equal to the full width of the beam pulse to be injected into the RF accelerator waveguide. Once injected into the waveguide, the macropulse of electrons is then modulated by the RF input into the waveguide to create a series of micropulses at the RF frequency. As an example, in a typical medical linear accelerator, a macropulse is approximately 5 xcexcS long. Typical micropulses generated from these 5 xcexcS pulses are approximately 30 pS long (that is, there are many thousand micropulses created from each micropulse input into the linear accelerator).
This process of pre-bunching makes it possible to improve the efficiency of a linear accelerator. However, it can be difficult and costly to produce an efficient and well-tuned pre-buncher or input cavity of a waveguide. In particular, the amount of current generated by a cathode of a typical electron gun must be very large because much of the gun current is lost between the electron gun and the linear accelerator. Due to inefficiencies in pre-bunchers or input cavities, much of the gun current is stopped or reversed by the accelerator RF and returned to the gun. This current which is stopped or returned to the gun increases heat at the cathode. Unless the effect of this returned current is properly factored into the cathode heat system design, the increased heat will reduce the life of the cathode and also increase barium evaporation at the cathode which can result in the generation of increased dark current.
Further, the use of a pre-buncher or input cavity requires the manufacture, assembly, and design of an additional cavity or structure at the input end of the accelerator. This can increase the cost of design and manufacture and further complicates the tuning, focusing and control of these complex devices.
It would be desirable to provide an electron gun which is operable to produce bunches of charged particles at a desired frequency without the use of a separate pre-buncher or input RF cavity. It would further be desirable to provide an electron gun for which the frequency and amount of the bunches of charged particles can be varied based on the needs of a particular application. Preferably, such a device could be used with existing accelerators without substantial modification to the accelerator tube.
To alleviate the problems inherent in the prior art, embodiments of the present invention provide an improved electron gun which produces bunches of charged particles at a desired frequency without use of a separate pre-buncher or input RF cavity. The present invention is not limited to the disclosed preferred embodiments, however, as those skilled in the art can readily adapt the teachings of the present invention to create other embodiments and applications.
According to one embodiment of the present invention, an electron gun includes a cathode, electrically coupled to operate as a source of charged particles, and a grid, positioned apart from the cathode. The grid and the cathode are electrically coupled to a grid voltage source and to a radio frequency (RF) source. The grid voltage source places the grid at a first potential, and the radio frequency source places the grid at a second potential selected to produce groups of the charged particles. The groups of charged particles are produced with each period of a signal received from the RF source.
According to one embodiment, the RF source is adapted to receive a signal from a primary RF source. The primary RF source provides an RF signal to an accelerator and to the RF source. According to one embodiment, the RF signal is attenuated and/or phase shifted before it is provided to the grid of the electron gun. According to one embodiment of the invention, the duration of the bunches of electrons is selected by variably attenuating the signal.
In a further embodiment of the present invention, a method for generating an electron beam is provided where a first RF signal is generated for input into a linear accelerator. A modified RF signal is generated. Electrons are caused to be emitted from an emitting surface of a cathode. A D.C. bias is applied to the cathode and to a conductive grid, the conductive grid positioned apart from the emitting surface of the cathode. The modified RF signal is applied to the cathode and to the conductive grid, where the modified RF signal is selected to accelerate groups of the electrons, each of the groups having a duration. The groups of electrons are accelerated into the linear accelerator by applying an anode bias to an anode positioned apart from the cathode.